1. Field of the Invention
This disclosure provides processes and apparatus for the production of chlorine dioxide. More particularly, the present invention is directed to processes and apparatus for production of a high concentration aqueous solution of chlorine dioxide without producing undesirable side products such as free chlorine or chlorous acid. The chlorine dioxide according to the present invention can be produced on site and can be used, inter alia, as a disinfectant in the treatment of water and wastewater.
2. Description of Related Art
Chlorine dioxide is a strong oxidant which has been receiving increased attention as an alternative to chlorine for the disinfection and taste/odor (T/O) control of water and waste water. The molecular formula of chlorine dioxide is expressed as ClO.sub.2. As implied from its chemical formula, it has the disinfecting properties of both chlorine and oxygen. Moreover, chlorine dioxide exhibits promise of good disinfection performance without the disadvantages of forming large quantities of undesirable chlorinated byproducts since it does not react with hydrocarbons to form chlorinated hydrocarbons.
Chlorine dioxide (ClO.sub.2) first was discovered in 1811 in the form of a greenish yellow gas by Sir Humphrey Davy, by reacting potassium chlorate (KClO.sub.3) with hydrochloric acid (HCl). It later was found that ClO.sub.2 could be used in a dilute acetic acid (CH.sub.3 COOH) solution for the bleaching of paper pulp. Even though the outstanding disinfecting properties of chlorine dioxide have been constantly noted, its practical application was hampered due to the lack of a safe and economical way of synthesizing it. In the 1930's, the Mathieson Alkali Works developed the first commercial process for making ClO.sub.2, from sodium chlorate (NaClO.sub.3) via sodium chlorite (NaClO.sub.2).
In 1944, the Niagara Falls Water Treatment Plant No. 2 was the first U.S. water treatment facility to use ClO.sub.2. ClO.sub.2 was used to treat a potable water supply for taste and odor (T/O) control, especially T/O from phenolic compounds. Industrial waste water streams commonly contain phenolic compounds and ammonium salts. Other U.S. plants soon after adopted ClO.sub.2 for water treatment; for example, Greenwood, S.C., Tonawanda, N.Y., Lockport, N.Y., etc. By 1958, over 150 municipal water plants adopted ClO.sub.2. In 1978, a survey showed that 84 U.S. plants were using ClO.sub.2 and that most of these plants were older plants. In Europe, over 500 water treatment plants were believed to be using ClO.sub.2 for water treatment in 1978.
In the 1990's, the U.S. EPA recommended that, as a part of the reauthorization of the Clean Water Act, a study should be undertaken to develop a strategy to prohibit, reduce, or find substitutes for the use of chlorine and chlorinated compounds. In recent years, free chlorine (Cl.sub.2) has been criticized by environmentalists, even though it is one of the most heavily used chemicals in various chemical and environmental applications. The disadvantages associated with using free chlorine can be summarized as follows:
(1) It is quite reactive with various substances including water, ammonia, and hydrocarbons; PA1 (2) Even with water, it reacts to produce hydrochloric acid and hypochlorite; PA1 (3) Solubility in water is relatively low making it difficult to adequately disinfect without affecting the vapor space above; PA1 (4) Chlorine is not effective in taste and odor (T/O) control, due to its low water solubility, own pungent odor, and acidic reaction; and PA1 (5) It is produced only as a bulk chemical commodity. A small batch capability does not exist, because on-site generation of chlorine is commercially unattractive. This makes chlorine unsuitable for waste water treatment. PA1 simplified reproducible chemistry; PA1 the use of weaker acids and less acid than the processes of the related art; PA1 no chlorine or chlorous acid produced as byproducts; PA1 mild reaction conditions; PA1 a high selectivity of chlorine dioxide and minimum selectivity of undesired byproducts; PA1 an increased reaction rate to decrease total reaction time; PA1 a low capital investment requirement; and PA1 a high yield of chlorine dioxide.
For at least these reasons, the replacement of chlorine with other chemicals such as chlorine dioxide has been the subject of a great deal of interest in the recent years.
Chlorine dioxide is known to be an excellent disinfectant as well as a strong oxidizing agent. Its bactericidal, fungicidal, algicidal, bleaching, and deodorizing properties are well documented in various sources of literature. Chlorine dioxide is soluble in water at room temperature (20.degree. C.) to about 2.9 grams ClO.sub.2 per liter of water at 30 mmHg partial pressure of ClO.sub.2, or 8 grams per liter at 80 mmHg partial pressure. ClO.sub.2 is approximately 5 times more soluble in water than chlorine gas (Cl.sub.2). ClO.sub.2 is much more soluble in water than oxygen (O.sub.2) which only has 9.2 mg solubility per liter of water. The presence of chlorine dioxide in water is very easily detected by a color change. The color in water changes from yellowish green to orange red as the concentration of ClO.sub.2 increases in water. At low temperatures, chlorine dioxide dissolves in water to a substantially greater extent due to lower vapor pressure, e.g., 12 g/L at 60 mmHg of partial pressure and 10.degree. C. FIG. 1 shows the solubility of ClO.sub.2 in water as a function of temperature. It can be seen that a lower temperature is preferred for higher aqueous solubility.
The boiling point (b.p.) of liquid ClO.sub.2 is 11.degree. C. and the melting point (m.p.) is minus 59.degree. C. Gaseous ClO.sub.2 has a density of 2.4 (when taking air as 1.0) and its molecular weight is 67.45 g/mol, i.e., it is a heavier gas than air. If chlorine dioxide is leaked into the air, it will tend to stay low, near the ground, then gradually diffuse into the atmosphere.
Chlorine dioxide (ClO.sub.2) differs from Cl.sub.2 in that ClO.sub.2 does not react with water or ammonia. Also, unlike chlorine, ClO.sub.2 does not produce chlorinated hydrocarbons after reacting with hydrocarbons. In general, ClO.sub.2 is less corrosive to most metallic and nonmetallic substances than chlorine, which is an important advantage.
It is also notable that ClO.sub.2 is quite volatile and therefore can be removed easily from aqueous solutions with minimum aeration. Concentrations of ClO.sub.2 in air above 11% can be mildly explosive. Due to the chlorine dioxide's relative instability and volatility, storage and transportation seem intuitively less economical, even though it is conceivable to store it in a compressed container. In this regard, the strategy of ClO.sub.2 production can be two-fold, viz., either on-site production or high purity compressed ClO.sub.2.
There have been several, but primarily three basic processes developed for the synthesis of ClO.sub.2 that have been commercially applied to water treatment operations. All three processes involve sodium chlorite (NaClO.sub.2) as one of the starting raw materials. The basic process chemistry of the three processes are discussed below.
Process 1: Process with Sodium Chlorite and Strong Acid
In this process, a strong acid is used along with sodium chlorite. The strong acid normally is hydrochloric acid or sulfuric acid. Using hydrochloric acid, the reaction stoichiometry is: EQU 5 NaClO.sub.2 +4HCl.fwdarw.4ClO.sub.2 +5 NaCl+2 H.sub.2 O (R1)
As shown, for every mole of ClO.sub.2 (i.e., 67.45 grams of ClO.sub.2) to be produced, the reaction requires 1.25 moles of sodium chlorite (i.e., 113.06 grams of NaClO.sub.2) and another mole of hydrogen chloride (i.e., 36.45 grams of HCl), assuming there is 100% conversion efficiency, which is impossible to expect from this process. Furthermore, 1.25 moles of sodium chloride salt (i.e., 73.13 grams of salt) are a by-product of each mole of chlorine dioxide produced.
Alternatively, chlorine dioxide can be produced using sulfuric acid, according to the following reaction: EQU 10 NaClO.sub.2 +5 H.sub.2 SO.sub.4 .fwdarw.8 ClO.sub.2 +5 Na.sub.2 SO.sub.4 +2 HCl+4 H.sub.2 O (R2)
A very similar situation to the above HCl case is expected, i.e., requirement of strong acid and production of sodium sulfate salt. Again, strong acids are disadvantageous due to their corrosive behavior, and the formation of large quantities of alkali metal salts such as sodium salts is disadvantageous because such salts typically must be removed by extraneous purification techniques. Of these two options, the HCl route seems to be more popular.
Process 2: Process with Sodium Chlorite and Gaseous Chlorine
This process uses gaseous chlorine along with sodium chlorite. The process operates in two stages, first beginning with the formation of an aqueous hypochlorous acid, i.e., EQU Cl.sub.2 +H.sub.2 O.fwdarw.HOCl+HCl (R3)
The intermediate product, hypochlorous acid (HOCl), in turn reacts with sodium chlorite to form chlorine dioxide (ClO.sub.2), i.e., EQU HOCl+HCl+2 NaClO.sub.2 .fwdarw.2 ClO.sub.2 +2 NaCl+H.sub.2 O(R4)
The stoichiometric reaction, which is a summation of the two, becomes EQU Cl.sub.2 +2 NaClO.sub.2 .fwdarw.2 ClO.sub.2 +2 NaCl (R5)
This process, however, involves chlorine and its attendant disadvantages. Furthermore, the process also involves an unstable intermediate, HOCl, thereby substantially limiting the process efficiency. The formation of chlorous acid (HOCl) can be very hazardous at elevated temperatures due to its volatility and propensity to release toxic chlorine gas. A fairly sizable amount of salt production also occurs.
Process 3: Process with Sodium Chlorite and Sodium Hypochlorite
In this process, sodium hypochlorite (NaOCl) is used as a raw material along with sodium chlorite: EQU NaOCl+HCl.fwdarw.NaCl+HOCl (R6) EQU HCl+HOCl+2 NaClO.sub.2 .fwdarw.2ClO.sub.2 +2 NaCl+H.sub.2 O(R7)
This process has several similarities with the earlier two processes, i.e., involvement of hypochlorous acid (HOCl), use of strong hydrochloric acid as a raw material, byproduct salt formation, etc. Due to the presence of sodium hypochlorite, the process intuitively seems to provide more bleaching possibility.
For much larger users of ClO.sub.2, such as for bleaching paper pulps, chlorine dioxide demands are much higher than in water treatment plants. In such applications, the sodium chlorite production step can become a part of the entire ClO.sub.2 generation process. This process has major drawbacks of using (1) sodium hypochlorite (NaOCl) that is expensive, and (2) hydrochloric acid (HCl) that is strongly corrosive.
U.S. Pat. Nos. 4,925,645 and 5,122,282 describe a process for the production of chlorine dioxide, and a method of treating water and/or wastewater using chlorine dioxide, respectively. According to these patents, the process of making chlorine dioxide includes the steps of combining lactic acid or citric acid with sodium chlorite or alkaline earth metal to yield a salt of an acid and chlorous acid. They state further that the products produced from the methods described therein include, inter alia, chlorine dioxide, as well as free chlorine, chlorous acid and chloric acid. The reaction mechanisms described therein are as follows: ##STR1##
The chlorine dioxide along with co-produced chlorine gas and chlorous acid are described by these patents as having the biocidal effect. As described above, the co-production of these chemicals is hazardous and disadvantageous. In addition, these patents claim to produce a chlorine dioxide solution that is stable for over 30 days. The present inventor has found that aqueous solutions of chlorine dioxide cannot remain stable under normal conditions for 30 days using the guidelines provided in those patents. Due to the inherent instability of aqueous chlorine dioxide solutions, the present processes and apparatus are intended to be used to manufacture chlorine dioxide on-site.
U.S. Pat. No. 4,084,747 describes a chlorine dioxide germ killing composition which is produced by contacting an acid material with sodium chlorite in an aqueous medium with a pH of less than 7. The '747 patent states that the lactic acid can be used in conjunction with other organic and inorganic acids. Disadvantages are described, however, when combinations of lactic acid and additional acids are used, compared to lactic acid alone. In addition, the '747 patent discloses a process by which undesirable by-products such as sodium lactate and chlorous acid are produced and hence, must be removed. The chlorine dioxide is used in a concentration of from 100 to 500 ppm up to 2700 to 3300 ppm.
U.S. Pat. No. 4,585,482 describes a process for producing a biocidal composition which liberates chlorine dioxide. The composition releases the chlorine dioxide when the pH is lowered to less than about 7 by an organic acid generating polymer. Thus, these documents describe a process for producing chlorine dioxide which involves either (i) the use of free chlorine or corrosive strong acids such as HCl and H.sub.2 SO.sub.4 or (ii) the co-production of hazardous by-products such as free chlorine, chlorous acid, and the like.
Conventional chlorine dioxide solutions prepared using methods disclosed in the aforementioned prior art suffer from the drawbacks that they produce undesirable by-products. In addition, the reactions involved using the methods described, i.e., merely mixing the reactants in a reactor usually at ambient temperatures and pressures, or slight modifications thereof, proceed at commercially unacceptable slow rates and produce relatively low concentrations of chlorine dioxide, i.e., on the order of less than about 5,000 mg/l of chlorine dioxide.
Thus, there exists a need to provide an economic and efficient method for producing chlorine dioxide that does not also produce hazardous by-products (e.g., chlorine or chlorous acid), as well as substantial amounts of unusable salts (e.g., sodium chloride, sodium lactate). There also exists a need for a method of producing chlorine dioxide that does not suffer from the aforementioned disadvantages with respect to the slow rate of reaction and the low concentrations of chlorine dioxide. Lastly, there exists a need for an apparatus capable of accomplishing such methods.